Recent advances in the fields of microelectronics have enabled the production and use of components which operate at microwave and millimeter wave frequencies. These components include power amplifiers that may operate in complex telecommunication systems or in relatively simple household appliances like microwave ovens. One type of previous power amplifier is called a "push-pull" amplifier. FIG. 1 presents a simplified diagram of a push-pull amplifier PPA from the text by John P. Steiner entitled An Illustrated Guide to Basic Electronics published by Prentice-Hall in 1984. The amplifier PPA includes two transistors Q1 and Q2. An alternating current input is applied to the primary coil of a transformer T. The signal from the secondary coil supplies an input to transistors Q1 and Q2. Since the input signals to the transistors are always in opposite polarity, one transistor conducts while the other is cut off. This device is called a push-pull amplifier because each of the pair of transistors works in opposition to amplify the input signal. See Steiner, pp. 222-223. The amplifier shown in FIG. 1 is a push-pull amplifier in the strictest sense. This means that the current that flows into the control terminal of the first device is exactly equal to and opposite to the current that flows out of the control terminal of the second device. These currents also flow through the common terminal that couples the two devices. This particular example of a PPA has a single-ended output, but this is not a requirement. The output could be balanced just as well.
Several recent papers on microwave push-pull amplifiers are listed below. These papers describe amplifiers that are designed to operate in the microwave and millimeter wave frequencies. None of these amplifiers, however, is push-pull in the strict sense defined above.
FET-Based Planar Circuits for Quasi-Optical Sources and Transceivers by Joel Birkeland and Tasu Itoh, published in IEEE Transactions on Microwave Theory and Techniques, Vol. 37, No. 9, p. 1452, September 1989. Birkeland et al. disclose transmitter and transceiver microwave circuits.
Six to Eighteen GHz Single-Ended and Push-Pull MMIC Amplifiers for High-Gain Modules by R. Ramachandran et al., published in IEEE 1988 Microwave and Millimeter-Wave Monolithic Circuits Symposium, p. 15, 1988. The authors discuss two single-ended amplifiers connected in push-pull fashion which are designed for the highest possible gain efficiency.
Complementary, HBT Push-Pull Amplifier by Selective MBE by K. W. Kobayashi et al., published in IEEE Microwave and Guided Wave Letters, Vol. 2, No. 4, p. 149, April, 1992. The authors disclose a GaAs heterojunction bipolar transistor amplifier that operates at microwave frequencies.
A Wide-Band Push-Pull Amplifier Upgrades IP2 by M. C. Tsai, published in 1990 IEEE MTT-S Digest, p. 511, IEEE, 1990. Tsai describes a push-pull amplifier that utilizes Lange coupler baluns.
New Broadband Balun Structures for Monolithic Microwave Integrated Circuits by B. J. Minnia and M. Healy, published in 1991 IEEE MTT-S Digest, p. 425, IEEE, 1991. Minnia and Healy disclose two passive balun structures that are intended for use in MMIC push-pull power amplifiers.
Broad-Band Push-Pull Power Amplifier by Sachihiro Toyoda, published in 1990 IEEE MTT-S Digest, p. 507, IEEE, 1990. Toyoda describes a broad-band phase inverter that is necessary for use in his push-pull amplifier.
A Ku-Band Oscillator Subsystem Using a Broadband GaAs MMIC Push-Pull Amplifier/Doubler by Robert Martin and Fazal Ali, published in IEEE Microwave and Guided Wave Letters, Vol. 1, No. 11, p. 348, November, 1991. Martin and Ali describe a voltage controlled oscillator subsystem that is used as a frequency doubler.
In a practical sense, conventional push-pull amplifiers provide higher gain due to lower common lead inductance. The overall efficiency of the amplifier is improved, and the higher gain supplied by each amplifier stage enables circuit designers to employ fewer stages to achieve a given level of gain. Since the number of stages is reduced, variations in the gain of multistage amplifiers due to temperature are reduced. Other benefits realized by minimizing the number of amplifier stages include enhanced reliability and lower manufacturing cost.
Compared to other types of amplifiers, push-pull amplifiers also offer the desirable characteristics of higher input and output impedance. This is a result of the fact that push-pull devices have their input and outputs connected in a series configuration. These features result in lower loss due to relatively lower transformation ratios, improved efficiency and greater bandwidth. The push-pull circuit also provides even harmonic suppression which not only can augment the output power, but also requires less filtering.
Conversely, push-pull designs are usually much more difficult to implement. In general, the current state of the art does not include high-gain yet low-cost push-pull amplifiers (in the strict sense) that operate at frequencies above 2 GHz. Previous push-pull circuits intended for use above the 2 GHz frequency range have generally been constructed using two single-ended, non push-pull amplifiers combined with a device that introduces a 180 degree phase shift. The only advantages supplied by this configuration are those associated with harmonic cancelling.
The development of power microwave amplifiers has presented a major challenge to designers in the field of microelectronics. The development of a low cost, highly efficient, high gain power amplifier which is capable of operating at microwave and millimeter wave frequencies would constitute a major technological advance and would satisfy a long felt need in the electronics and telecommunications industries.